During previous years of this project, a set of flux-ratio equations was derived (based on previous work of others) which described transport processes in terms of the relationship between bulk diffusion (or permeability) coefficient to tracer diffusion (or permeability) coefficient. The equations have been validated for active and passive transport of sodium ions in toad bladder, in the presence and absence of gradients of potential, concentration, or both. In the proposed work, the utility of these equations in determining the mechanism by which aldosterone stimulates short-circuit current and active sodium transport will be explored, and in an attempt to demonstrate that the membrane property Q is not equal to unity for non-electrolyte transport, they will be used to analyze the fluxes of urea, mannitol and thiourea across toad bladder. BIBLIOGRAPHIC REFERENCE: Chen, J.S. and M. Walser. Effect of transepithelial concentration gradient on the passive fluxes of sodium across toad bladder. J. Membrane Biology 27:381-391, 1976.